This invention relates to arc discharge lamps and more particularly to metal halide lamps. Still more particularly it relates to metal halide lamps that will operate at low pressures (i.e., about 1.5 atmospheres and thus capable of operation without a shroud) and with a minimum amount of mercury allowing the expired lamps to be conventionally landfilled.
Many arc discharge lamps contain elemental mercury. During lamp operation, chemical reactions take place that convert some of the elemental mercury to salts or compounds, such as mercuric oxide (HgO), that are water soluble. There is a growing concern that a waste stream resulting from the disposal of fluorescent lamps may leach excessive amounts of this soluble form of mercury (Hg) into the environment. An acceptable method of measuring the amount of soluble mercury which may leach from the waste stream resulting from the disposal of discharge lamps is described in the Toxicity Characteristic Leaching Procedure (TCLP) prescribed on pages 26987-26998 of volume 55, number 126 of the Jun. 29, 1990 issue of the Federal Register. The lamp to be tested is pulverized into granules having a surface area per gram of materials equal to or greater than 3.1 cm2 or having a particle size smaller than 1 cm in its narrowest dimension. The granules are then subject to a sodium acetate buffer solution having a pH of approximately 4.9 and a weight twenty times that of the granules. The buffer solution is then extracted, and the concentration of mercury is measured. At the present time, the United States Environmental Protection Agency (EPA) defines a maximum concentration level for mercury to be 0.2 milligram of leachable mercury per liter of leachate fluid when the TCLP is applied. According to the present standards, an arc discharge lamp is considered nonhazardous (and thus available to be conventionally landfilled) when less than 0.2 milligram per liter of leachable mercury results using the TCLP. Lamps that have leachable mercury concentrations above the allowable limit must be especially disposed of through licensed disposal operations. Disposal operators charge a fee for disposal of lamps that are not within the EPA""s limits. Therefore, customers must pay extra costs to dispose of these lamps. Customers of arc discharge lamps generally desire not to contend with disposal issues regarding mercury levels, and therefore some customers specify only those lamps, which pass the TCLP standard.
Additionally, discharge amps generally designated as metal halide lamps often have arc tubes that operate at high pressure (i.e., up to 8 atmospheres) and are required to be shielded to offer protection in the event of an arc tube failure. Such shielding is usually accomplished by the provision of a tubular shroud, which surrounds the arc tube. While workable, the shroud adds expense to the cost of the lamp.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the operation of arc discharge lamps.
It is another object of the invention to provide a low-pressure arc tube and thus reduce the cost of arc discharge lamps by eliminating shrouds.
Yet another object of the invention is the provision of an arc tube, and thus a lamp, that can pass required TCLP testing.
Still another object of the invention is the provision of a low pressure lamp that will work on existing ballasts.
These objects are accomplished, in one aspect of the invention, by the provision of a ceramic arc tube for a metal halide lamp. The arc tube has a bulbous body with a hollow center portion and opposite ends; each of the opposite ends having a cylindrical, terminal-receiving section extending therefrom. The bulbous body has an aspect ratio  less than 5 and an outer surface to inner surface ratio, measured in square units, of less than 1.5. As used herein the term xe2x80x9caspect ratioxe2x80x9d refers to the internal length of the arc chamber divided by the internal radius diameter.
These objects are additionally accomplished, in another aspect of the invention, by the provision of a shroudless metal halide lamp that comprises an hermetically sealed outer envelope terminating in a base at one end and having a ceramic arc tube operatively mounted within the outer envelope. The arc tube comprises a bulbous body having ends and an electrode hermetically sealed in each of the ends and extending internally and externally thereof, the electrodes being operatively connectable to a source of electrical power through the base of the lamp. The bulbous body has an aspect ratio of less than 5 and an outer surface area to inner surface area ratio, measured in square units, of less than 1.5. An arc generating and sustaining medium is contained within the bulbous body, the medium containing additives to generate a particular color emission when the arc tube is operating and an amount of mercury sufficient to provide the arc tube with an operating pressure of about 1.5 atmospheres or less.
The low operating pressure allows the lamp to operate without an internal shroud to protect the arc tube and the low aspect ratio of less than 5 lets the lamp operate with a quantity of mercury small enough for the lamp to pass the TCLP test.